These are the zombies who sprint very fast, and to your alarm, can climb over obstacles. They can deliver several quick attacks with their rotting claws. However, they are very weak (compared to Walkers) and easily killed.

The skull faced zombie with missing forearms and hands. The Butcher was an upgraded version of the Infected (being tougher) but it could still be knocked down with a strong enough hit, and proved to be less fearsome that the in-game notes led you to believe.

The zombie in the straight jacket. If it gets sight of you, it will charge. The Ram is armored everywhere (including the head, although it takes more damage there) except for its back. Shooting the back of the Ram is more effective than shooting its head.

A zombie who runs like the Infected (but cannot climb). They emit a loud shriek that deafens your character (preventing them from doing anything except covering their ears) and hits as hard as the Infected.